1 April 1917: birth of Sydney Newman, without whom etc etc.
1 April 1926: birth of John Scott Martin, Dalek operator and rubber-suited monster extraordinaire.
ii) broadcast anniversaries
1 April 1967: broadcast of fourth episode of The Macra Terror. The Doctor floods the mines with oxygen and then blows them up, killing the Macra.
1 April 1972: broadcast of sixth episode of The Sea Devils. The Doctor sabotages the Sea Devils' base and it explodes killing them all; he and the Master escape.
iii) date specified in canon
1 April 1963: setting of First Doctor novella Time and Relative (2001), by Kim Newman.